Crystalline materials are classified by the Structure Commission of the International Zeolite Association according to the rules of the IUPAC Commission on Zeolite Nomenclature. According to this classification, framework type zeolites and other crystalline microporous crystalline materials, for which a structure has been established, are assigned a three letter code and are described in the Atlas of Zeolite Framework Types, Sixth Revised Edition, Elsevier (2007).
One known crystalline material for which a structure has been established is the material designated with MFI framework-type, most notably including ZSM-5. Crystalline ZSM-5 and its conventional preparation using tetrapropylammonium cations as a structure directing agent, are taught by U.S. Pat. No. 3,702,886 and RE 29,948, the entire disclosures of which are incorporated herein by reference. Conventional ZSM-5 has a distinctive X-ray diffraction pattern which distinguishes it from other known crystalline materials and is a highly versatile catalyst useful in a variety of organic conversion reactions.
Another known crystalline material structure is MEL, also known as ZSM-11, which is described in detail in U.S. Pat. No. 3,709,979, the entire disclosure of which is incorporated herein by reference. MFI and MEL framework-type crystalline materials have similar structures and are frequently co-produced in zeolite synthesis processes as intergrown or disordered materials.
For some acid-catalyzed reactions over zeolites, it is beneficial to reduce diffusion lengths of the reagent and/or product molecules by employing a zeolite with a reduced crystal size and hence an increased external surface area. This may have the effect of reducing the shape selective effects of the zeolite, but for reactions that require only strong activity this may not be important. For example, the increased external surface area permits reactions with larger molecules that cannot enter the pores of the zeolite. In addition, in some processes it has been observed that the rate of deactivation is reduced when the external surface area of the ZSM-5 zeolite is increased. See, for example, M. Choi et al, Nature 461 (2009) 246-249 J. Kim, M. Choi, R. Ryoo, J. Catal. 269 (2010) 219-228). Increased external surface area has also been reported to improve the propylene yields in methanol conversion. See, for example, M. Firoozi et al. Catal. Commun. 10 (2009) 1582-1585.
An example of small crystal ZSM-5 is disclosed in U.S. Pat. No. 5,240,892, in which the ZSM-5 is in the form of platelets having first and second major dimensions of at least about 0.05 micron, preferably at least about 0.1 micron, and a minor third dimension of less than about 0.02 micron, preferably less than about 0.01 micron. The ZSM-5 has a mesitylene sorption capacity of at least 3.0 weight % and is produced using precipitated silica as the silica source either in the absence of an organic directing agent or using n-propylamine as the directing agent.
In addition, in Chem. Mater. 21 (2009) 641-654, D. Serrano et al. claim synthesizing ZSM-5 crystals as small as 5 to 10 nm using a dual template of tetrapropylammonium (TPA) ions and phenylaminopropyltrimethoxysilane. In this method, the silanizing agent is introduced after the synthesis gel is pre-heated for short periods of time before the onset of zeolite crystallization. FIG. 1 of Serrano et al. shows a schematic representation of the crystallized products, whereas FIG. 2 shows TEM images of the product. Although these TEM images show very small particles, the peaks in the powder XRD of the product from this work are consistent with crystals exceeding 5 to 10 nm in their dimensions.
Ryoo and coworkers have reported in “Stable single-unit-cell nanosheets of zeolite MFI as active and long-lived, catalysts”, Nature, 461, 246-249 (10 Sep. 2009), the synthesis of a single unit cell-thick version of ZSM-5 by using a single templating agent composed of a 22-carbon atom alkyl chain and two quaternary ammonium groups separated by a methylene chain of 6 carbon atoms. Here the quaternary ammonium groups are located within the single-unit cell nanosheets, which are separated from one another by the long alkyl chains. FIG. 3 of Ryoo et al. shows a schematic of the unilamellar and multilamellar version of the ZSM-5 crystals that are a single unit cell in thickness.
U.S. Patent Application Publication No. 2015/0298981 describes a crystalline material designated as EMM-20 and having the framework structure of ZSM-5 comprising crystals having an external surface area in excess of 100 m2/g (as determined by the t-plot method for nitrogen physisorption) and a unique X-ray diffraction pattern. The crystalline material may be synthesized in the presence of an organic structure directing agent (Q) selected from one or more of 1,4-bis(N-pentylpyrrolidinium)butane dications, 1,5-bis(N-pentylpyrrolidinium)pentane dications, or 1,6-bis(N-pentylpyrrolidinium)hexane dications.
In 2012, Zhang et al. reported the synthesis of a “self-pillared” MFI or WI/MEL-type materials using tetrabutylphosphonium hydroxide (TBPOH) and/or tetrabutylammonium (TBAOH) as the structure-directing agent (SDA). See “Synthesis of Self-Pillared Zeolite Nanosheets by Repetitive Branching”, Science, 336 (2012), 1684-1687. TEM of the reported products shows a morphology composed of thin sheets of zeolite that are about 1 unit cell in width. The thin sheets interpenetrate one another at right angles to form a cross-hatched network of crystals with mesopores between them that are 2-7 nm in size. The resulting “house-of-cards” arrangement of the nanosheets creates a permanent network of 2- to 7-nanometer mesopores, which, along with the high external surface area and reduced micropore diffusion length, account for higher reaction rates for bulky molecules relative to those of other mesoporous and conventional MFI zeolites. The publication reported only materials with gel Si/Al 75.
Despite these advances, a need still exists for new ultra-small crystal forms of MFI and MEL framework-type crystalline materials with higher external surface areas and to extend the Si/Al range over which these materials can be synthesized.